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• The atmosphere is a mixture of gases.
• The atmosphere is mostly nitrogen (approximately 80%) and oxygen (approximately 20%). The remaining gases are found in much smaller proportions, such as carbon dioxide and water vapour.
• Oxygen in the atmosphere is essential for all living things to stay alive, and for the combustion (burning) of fuels.

Watch the video to learn about the mixture of gases in the atmosphere.

The Earth’s atmosphere is the relatively thin layer of gases surrounding the planet. The atmosphere is held close to Earth by gravity, but the higher you go away from the Earth’s surface, the thinner the air. Because of this, it’s impossible to say how high the atmosphere extends above Earth accurately. However, most scientists use around 100 km when describing where the atmosphere ends and space begins.

The three gases with the highest percentages in the atmosphere are all elements:

• 78% nitrogen, N₂
• 21% oxygen, O₂
• 0.9% argon, Ar

These three gases make up 99.9% of the atmosphere.

The remaining gases are found in much smaller proportions. These include carbon dioxide and water vapour.

Did you know?

Even though it is hard to imagine that gases have much mass, the atmosphere extends so far above us, that the weight of the air above you is what causes air pressure (atmospheric pressure).

This weight of the air above is equivalent to the weight from a 1 kg mass pressing down on every square centimetre of your skin! You don’t notice this pressure because it’s always been there and it is balanced by the pressure of the liquids inside your body.

All living things on Earth, including humans, use oxygen from the atmosphere for respiration to stay alive.

Respiration is the chemical process that occurs inside cells to release energy from glucose when it reacts with oxygen.

The word equation is:

Glucose + oxygen → carbon dioxide + water

Plants and some bacteria use carbon dioxide from the atmosphere for photosynthesis.

Photosynthesis is the chemical process when carbon dioxide and water react together, in the presence of light and chlorophyll, to produce glucose and oxygen.

The word equation is:

Carbon dioxide + water → glucose + oxygen

You can see from the equations that the reactants in one process are the products in the other process. Chemically speaking, these two processes are opposite.

Even plants and animals that live in the oceans use gases which have dissolved into the seawater from the air above it.

Combustion is another word for burning. Combustion happens when a fuel is heated and reacts with oxygen to release useful energy. Fuels are burned for lots of different purposes.

For example:

• Motor vehicles - the fuel is usually petrol or diesel
• In a domestic boiler for central heating and hot water - the fuel is usually natural gas (methane)
• For cooking - the fuel is often natural gas (methane)
• To keep warm - the fuel can be wood, coal or natural gas

For combustion to occur, there needs to be a fuel, heat and oxygen. This is represented by the fire triangle.

If one side of the fire triangle is removed, combustion cannot happen. In most situations, the oxygen for combustion comes from the atmosphere.

Most fuels contain carbon. When they are burned, carbon dioxide is produced. Human activities - like burning fossil fuels - over the past 200 years have increased the amount of carbon dioxide in the atmosphere, thickening the earth’s layer of greenhouse gases. As a result, the earth has been getting hotter. This is known as global warming.